We sought novel tumor-intrinsic determinants of tumor resistance to CAR T-cell therapy using an immunocompetent model of B-ALL in which human CD19 (hCD19)–transduced, mouse BCR1-ABL+ pre–B-ALL tumor clones were transferred into unconditioned C56BL/6 mice (7, 20). Here, BCR is linked to precursor B-cell acute lymphoblastic leukemia.